This invention relates to a process for producing paraffinic hydrocarbons useful as fuel from renewable feedstocks such as the glycerides and free fatty acids found in materials such as plant oils, fish oils, animal fats, and greases. The process involves hydrogenation, decarboxylation, decarbonylation, and/or hydrodeoxygenation in two or more reaction zones with each reaction zone having an associated feedstock. The reaction zones are operated at from about 80 to about 90 percent conversion of the triglycerides to paraffins. After separation of hydrogen and product gasses, the net liquid and hydrogen are passed to a finishing deoxygenation reactor to reach the desired overall conversion of glycerides to paraffins. Optionally hydroisomerization is conducted in yet an additional reaction zone.
As the demand for fuel such as diesel fuel, gasoline, and aviation fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing the fuel. One such source is what has been termed renewable sources. These renewable sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as inedible tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these sources is that they are composed of glycerides and Free Fatty Acids (FFA). Both of these compounds contain aliphatic carbon chains having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in the glycerides or FFAs can also be mono, di or poly-unsaturated. Some of the glycerides from the renewable sources may be monoglycerides or diglycerides instead of or in addition to the trigylcerides.
There are reports in the art disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Applicants have developed a process which comprises an optional pretreatment step, a first reaction zone to hydrogenate, decarboxylate, decarbonylate, and/or hydrodeoxygenate a first portion of the feedstock, a second reaction zone to hydrogenate, decarboxylate, decarbonylate, and/or hydrodeoxygenate a second portion of the feedstock, a separation zone to remove hydrogen and product gases from the combined products of the first and second reaction zones, and then a finishing zone to deoxygenate the net liquid from the separation zone to achieve the desired conversion of the glycerides and FFA to paraffins. The effluent from the finishing zone may be optionally hydroisomerized.
A volume ratio of recycle hydrocarbon to the first portion of the feedstock ranging from about 2:1 to about 8:1 provides a mechanism to increase the hydrogen solubility in the reaction mixture sufficiently so that the operating pressure of the process may be lowered. The range of successful volume ratios of recycle to the first portion of the feedstock is based upon the desired hydrogen solubility in the reaction mixture. There are various points in the process where the recycle stream can originate. The reaction zones may be operated at a pressure in the range of about 1379 kPa absolute (200 psia) to about 4826 kPa absolute (700 psia). Employing two reaction zones and a separate fresh feed stream to each reaction zone followed by a finishing zone allows for both the volume of recycle to be significantly reduced and for suitable catalyst life cycle times to be maintained in the first and second reaction zones.